Heretofore, power supplies for converting a high voltage AC input into a low voltage DC output have employed step-down transformers for converting the high voltage AC input into a suitable low voltage AC output at the power line frequency, generally 50 or 60 hertz. The low voltage AC output would then be converted into a low voltage DC output by rectifier diodes, filter capacitors and other filter components. Such transformers are heavy and bulky. Moreover, such transformers generate considerable heat which constitutes wasted energy. To accommodate a wide range of AC input voltages, the transformers have often been provided with primary windings having several taps connected to special input terminals which must be used to accommodate different voltages. The utilization of such transformers is particularly inefficient when the power supply is employed to operate small loads, such as the LEDs of an exit sign, and for recharging small storage batteries which may be used to provide backup DC power.